Making photoconductive target of selenium and sulphur



June 17, 1969 MAKING IPHOTOCONDUCTIVE TARGET OF SELENIUMAND SULPHUR s. A; B'YNUM Y g I if I I i i M I; l iH Original Filed June 28, 1965 IN VEN TOR.

A T7 OENE Y United States Patent 3,450,564 MAKING PHOTOCONDUCTIVE TARGET 0F SELENIUM AND SULPHUR Stanley A. Bynum, Dallas, Tex., assignor to General Electrodynamics Corporation, Garland, Tex., a corporation of Texas Continuation of application Ser. No. 467,268, June 28,

1965. This application Apr. 1, 1968, Ser. No. 718,012 Int. Cl. B4411 1 44 U.S. Cl. 117-201 2 Claims ABSTRACT OF THE DISCLOSURE A complex of sulfur and selenium in which the weight of sulfur is from about /2 to about 20% of the weight of selenium, the complex being in an amorphous form having a reduced transparency and having high resistance to conversion to crystalline form at temperatures up to about 70 C., said complex being produced by simultaneously depositing sulfur and selenium to form a coating, and heating the coating until it is converted to the less transparent form, and said complex being utilized as a target for a photoconductive pickup tube.

This invention relates to photoconductive devices, and more particularly it relates to photoconductive image pickup tubes of the type commonly known as the vidicon.

Television camera tubes employing photoconductive targets and known as vidicons are now well known in the art, having been described in an article in the May 1950 issue of Electronics magazine, and in a number of patents including, for example, Patent No. 2,745,032 to Forgue et a1. As described in the prior art, and as is well known, a vidicon camera tube consists of an electron gun and a target assembly contained in a glass envelope, usually about six inches long and one inch in diameter. The electron gun may be of the conventional type used in other types of television pickup tubes. The target assembly comprises a film of light-transparent, electrically conductive material on the face plate of the envelope, and a coating of photoconductive material deposited upon the electrically conductive film. The target and the gun are so arranged within the envelope that the electron beam from the gun scans the photoconductive surface of the target.

Photoconductive materials used for vidicon targets are electric insulators in the dark, but become electrically conductive when light is shined upon them. The conductivity is proportional to the amount of light striking the material, and is limited to the immediate area under the influence of the light. A number of different photoconductive materials are usable for various electronic devices, and one of the most widely used photoconductors is elemental selenium, in which, in fact, the phenomenon of photoconductivity was first observed, in 1873. Selenium has two common allotropic forms, both of which are widely used as photoconductors. The crystalline gray form, sometimes referred to as the metallic form, is a fair conductor even in the dark, and is used principally in photocells and rectifiers, and the like. The amorphous red form is used as a photoconductor for photoconductive television image pickup tubes and thin film detectors because it is characterized by a resistivity sufliciently high (approximately ohm-cm.) to permit charge storage operation. It is the latter allotropic form to which the invention described herein relates.

A major difliculty in the use of amorphous red selenium in photoconductive television image pickup tubes is that the material is unstable and will slowly convert to the more conductive crystalline form even at ordinary room temperature. At temperatures above about 45 C. the time required for such conversion is only a few hours. Thus where amorphous selenium has previously been used in television camera pickup tubes, it has been necessaiy to strictly limit the operating temperature of the tu e.

It is an object of this invention to provide a selenium base photoconductor which does not have the thermal instability of redamorphous selenium photoconductors previously used.

It is another object of this invention to provide a photoconductive coating, such as is used on a vidicon camera tube face plate, which is formed principally of amorphous selenium, but which has a high photosensitivity and is far more stable at relatively higher temperatures than is ordinary red amorphous selenium. Still another object of the invention is to provide a method for forming a photoconductive surface from selenium which provides a photoconductive surface having high resistivity in the dark and good sensitivity combined with a long life at ordinary operating temperatures.

Yet another object of the invention is to provide a vidicon camera tube which can be scanned at relatively long intervals and will still give good current to light response, and which has a long, useful life.

These and other objects of the invention are accomplished by forming a photoconductive surface from red amorphous selenium which has been combined with a small proportion of sulfur in such a manner that the selenium and sulfur form a complex, or an intermolecular compound, whereby the sulfur modifies the reaction of the selenium to temperature and makes the amorphousselenium more stable.

For a better understanding of the invention, reference is now made to the following description and to the single figure of the accompanying drawing, which shows a longitudinal and partly sectional view of one form of vidicon camera tube embodying the present invention.

The drawing shows a vidicon type camera tube, indicated generally by the reference numeral 10, which comprises an evacuated envelope 12 having an electron gun 16 in one end thereof. The electron gun 16 may be any of the known types of electron guns, and produces an electron beam directed toward the target electrode 18 in the other end of the envelope 12. The electron beam is focussed and scanned over the exposed surface of the target electrode by any conventional means (not shown).

The target electrode 18 is attached to a metal ring 19, made of a metal such as Kovar, which is sealed by means well known in the art to the edge of the target electrode and to the end of the envelope 12. The target electrode comprises a transparent face plate or substrate 20, prefe rably made of fused quartz or some similar material which has a low coeflicient of expansion. The face plate has applied thereto a layer 22 of a conductive material, which comprises a signal plate which in turn is covered by a layer 24 of a photoconductive material.

The transparent conductive layer or signal plate 22 may be made of tin oxide or a thin evaporated metal film such as gold, and may be deposited by any known technique such as spraying or evaporation. The signal plate should be highly electrically conductive and should be transparent to the particular radiant energy for which the device 10 is designed to respond.

The photoconductive coating 24 is made up of a particular form of red amorphous selenium which is stabilized by the combination therewith of a comparatively small amount of sulfur. Such a coating is prepared by first combining sulfur and selenium in such a Way that an amorphous atomically intimate and homogeneous mixture of selenium and sulfur is obtained. Such a mixture may be obtained, for example, co-melting the two elements in a protective atmosphere, or by co-precipitation, or by coevaporation of the two elements. It has been found that co-melting of the two elements in an evacuated tube at a temperature of around 400 C. is a convenient and reliable method of obtaining the desired homogeneous mixture.

After the mixture of selenium and sulfur is prepared it is fabricated into a photoconductive thin film lying on the signal plate of the target electrode. Such a thin film may be prepared by means well known in the art, such as, for example, by evaporating either in a high or low vacuum, or by settling, or by electrophoretic plating, or by other well known methods for applying such photoconductive layers. Such methods of depositing such coatings are well known in the art and do not form a part of this invention. However, good results have been achieved in forming the coating of this invention by evaporating the selenium sulfur mixture onto the conductive layer at a temperature of about 300 C. in a vacuum of approximately 0.1 mm. absolute pressure.

The photoconductor as thus formed comprises a red coating, amorphous by X-ray diffraction, which has good photoconductive sensitivity and has better heat stability than ordinary red amorphous selenium. However, the characteristics of the coating are substantially improved by following the coating process with a heat treatment which comprises holding the photoconductor at a temperature in the range of about 40 C. to about 100 C. for a time sufficient to convert the initial amorphous form to a second form which is also amorphous by X-ray diffraction, but which is characterized by a darker red color and a lower transparency than the original form. This second form of selenium-sulfur compound has been found to possess a higher photosensitivity than the first form, and also has excellent resistance to conversion into the gray crystalline form under exposure to moderate temperatue. The precise time required for the conversion from the first amorphous form to the second amorphous form will vary according to the particular temperature selected. It has been found that if the heat treatment is carried out at about 70 C., heat treatment for one hour is sufiicient to obtain substantial conversion to the second amorphous form.

The final product achieved by this process appears to be a mass of very small spherical particles of the second amorphous form with a small amount of the first amorphous form filling the interstices. The fact that the amorphous form obtained by the process of this invention is different from the usual red amorphous form is made evident by the performance of the photoconductive material. Vidicons which previously used red amorphous selenium for the photoconductor were limited to operations at temperatures under 45 C., and even at this temperature had a life of only a few hours before the red amorphous material would begin to convert to the gray crystalline form, which has such a low dark resistivity that it cannot be used in a vidicon. However, a vidicon which is provided with a photoconductor of the type prepared according to this invention has a much longer life even though operated at temperatures as high as 70 C. or greater. Apparently the addition of the sulfur to the selenium stabilizes the amorphous form of the selenium and prevents its ready conversion to the gray form upon heating.

It is not known precisely what proportion of sulfur is present in the coating applied to the signal plate of the target. However, it is thought likely that the proportions, of the materials in the photoconductive layer are very near the proportions which are in the mixture which is evaporated. It has been found that the advantageous results of this invention are obtained when the mixtures evaporated in forming the photoconductive layer comprise sulfur amounting to from about one-half percent to about twenty percent of the weight of the selenium present, although a preferred amount of sulfur is between about five percent to about ten percent of the weight of selenium.

Although preferred embodiments of the invention have been shown and described herein, the invention is not limited to such embodiments, but only as set forth by the following claims:

1. A process for preparing a photoconductive target comprising melting sulfur and selenium together in a vacuum to form an amorphous, atomically intimate and substantially homogeneous mixture containing about /z% to about 20% sulfur based on the weight of selenium,

evaporating said mixture in a vacuum and depositing it in a thin fil-m on a substrate to form a coating having the color of ordinary red amorphous selenium, and

without any intermediate heating sufficient to form crystalline selenium, heating the film at a temperature of from about 40 C. to about 100 C. for a period sufiicient to convert said coating to a less transparent amorphous form, free of crystalline selenium, said amorphous form having a darker red color, higher photosensitivity, a dark resistivity of at least about 10 ohm-cm, and high resistance to conversion to a gray crystalline form at temperatures up to about C.

2. A process for forming a photoconductive coating comprising depositing a mixture of selenium and about /2% to about 20% sulfur, based on the weight of the selenium, to form an amorphous photoconductive layer having the color of ordinary red amorphous selenium, and

without any intermediate heating sufiicient to form crystalline selenium, heating said layer at a temperature of from about 40 C. to about C. for a period sufiicient to transform said coating to a second, less transparent amorphous form, free of crystalline selenium, said amorphous form having a darker red color, higher photosensitivity, a dark resistivity of at least about 10 ohm-cm., and high resistance to conversion to a gray crystalline form at temperatures up to about 70 C.

References Cited UNITED STATES PATENTS 2,297,691 10/1942 Carlson. 2,662,832 12/1953 Middleton et a1. 2,962,376 11/1960 Schaffert. 3,234,020 2/1966 Stockdale.

ROBERT SEGAL, Primary Examiner.

U.S. Cl. X.R. ll7106; 313-94 

